1. Field
The present application relates to a programmable gain circuit and an amplification circuit.
2. Description of the Related Art
FIG. 1 illustrates a typical programmable gain circuit. A gain of the typical programmable gain circuit may be adjusted by switching operations of switching circuits 200.
As shown in FIG. 1, an input signal IN is inputted to one input terminal of an amplifier 100. An output signal OUT is outputted from an output terminal of the amplifier 100. A reference voltage VS is supplied to the output terminal of the amplifier 100 via a plurality of resistors Rs. Electric potentials of electrical connection points between respective resistors Rs become electric potentials obtained by dividing an electric potential difference between a voltage of the output signal OUT and the reference voltage Vs based on resistance values of respective resistors Rs.
As further shown in FIG. 1, the other input terminal of the amplifier 100 is coupled to the electrical connection points between respective resistors Rs via a plurality of switching circuits 200. The plurality of switching circuits 200 are controlled such that any one of the switching circuits 200 becomes conductive. When any one of the plurality of switching circuits 200 selectively becomes conductive, a voltage inputted to the other input terminal of the amplifier 100 may be adjusted. A gain of the amplifier 100 may be adjusted based on the adjustment of the voltage inputted to the other input terminal of the amplifier 100.
For example, if the switching operations of the plurality of switching circuits 200 are controlled with a three-bit control signal in the typical programmable gain circuit in FIG. 1, there may be three selectable gains.
Consequently, if a fine adjustment of the gain is required, the requirement causes an increase in the number of resistors and switching circuits. Moreover, the requirement causes an increase in the number of bits of the control signal that controls the plurality of switching circuits.
There are an N number of gains that may be selected with an N-bit control signal, in the typical programmable gain circuit in FIG. 1. The typical programmable gain circuit in FIG. 1 requires, for the fine adjustment of the gain, a control circuit that generates a multi-bit control signal, the switching circuits, and the resistors, wherein the number of the switching circuits and the number of the resistors are equal to the number of bits of the control signal. Consequently, there is a problem that an increase in circuit size is caused in the typical programmable gain circuit in FIG. 1.
FIG. 2 illustrates another typical programmable gain circuit.
As shown in FIG. 2, an input signal IN is inputted to one input terminal of an amplifier 300 via an input resistor R100. A reference voltage Vs is inputted to the other input terminal of the amplifier 300. An output signal OUT is outputted from an output terminal of the amplifier 300. The output terminal of the amplifier 300 is coupled to the one input terminal via a plurality of feedback resistors R200 to R500. For example, resistance values of the resistors R300 to R500 are set in the ratio of 1 to 2 to 4 (1:2:4).
As further shown in FIG. 2, switching circuits SW100 to SW300 are coupled in parallel to each of the resistor R300 to R500. The switching circuits SW100 to SW300 are switching-controlled based on a three-bit control signal. Since resistance values of the feedback resistors R200 to R500 may be adjusted in eight levels by the switching-control of the switching circuits SW100 to SW300, a gain of the amplifier 300 may be adjusted in eight levels with the three-bit control signal.
The gain is adjusted in 2N levels with an N-bit control signal in another typical programmable gain circuit in FIG. 2. In another typical programmable gain circuit in FIG. 2, a precise adjustment of the gain is very difficult due to ON-resistances of the respective switching circuits that affect the gain.
FIG. 3 illustrates a typical inverting amplification circuit. A gain G of the inverting amplification circuit is represented by Equation (1) if an input resistor is R600 and a feedback resistor is R700.G=OUT/IN=R700/R600  (1)
That is to say, the gain G is represented as a ratio of a resistance value of the input resistor R600 to a resistance value of the feedback resistor R700.
As shown in FIG. 4, a feedback resistor R800, to which a switching circuit SW400 is coupled in parallel, is coupled in series to a feedback resistor R700 for an adjustment of a gain G. If the switching circuit SW400 becomes conductive and an ON-resistance in the aforementioned conductive state is represented as RSW400, the gain G is represented by Equation (2).G=OUT/IN=(R700+RSW400)/R6  (2)
That is to say, the ON-resistance RSW400 of the switching circuit SW400 affects the gain G.
An amplification circuit discussed in Japanese Laid-open Patent Publication No. 1985-236509 is the amplification circuit which selects one of N level(s) in gain with an N-bit control signal. An amplifier discussed in Japanese Laid-open Patent Publication No. 1981-28524 corresponds to multiple input signals by switching feedback resistors and input resistors of the amplifier with analog switches. However, the number of the selectable feedback resistors and input resistors is equal to the number of the analog switches.
Consequently, it is very difficult to perform the fine adjustment of the gain even if the gain is selected by selection of the feedback resistor and the input resistor.